Book - School of Science and Technology

Noise dispersal 469wander across the plant room where noise can re-enter and be conveyed about thebuilding: such equipment should either be followed by an acoustically insulated duct orbe fitted immediately adjacent to the plant room wall. Figure 16.29 illustrates this point.An alternative to the absorption, or dissipative, type of attenuator is active attenuation.This is based upon the principle that two sound sources of the same frequency andamplitude, but displaced in time, or phase, together produce a reduced noise level.Therefore, introducing a microphone into a duct to measure the fan noise and feedingit through a phase change device, to be re-introduced into the duct by a loudspeaker, canprovide attenuation. This method is most effective at lower frequencies with longerwavelengths, which is the range where the dissipative type is least effective. A simplifieddiagram of the system is shown in Figure 16.30.Another application for attenuators occurs where a number of rooms are connected toa common duct system either inlet or extract, and where it is essential that speech orsounds in one room are not heard in other rooms. In Figure 16.31(a), a sound in room Ahas but a short path to rooms B and C. In Figure 16.31(b) the branch serving each room istreated acoustically, thereby avoiding any cross-talk. It will be noted that the `shunt-ducts'mentioned in Chapter 13 (Figure 13.11) may act to some extent as cross-talk attenuators.Privacy between spaces is also affected by the background noise level; the higher this levelAttenuatorBreak-inAttenuatorFigure 16.29 Noise `break-in' within a plant roomDuctLoud speakerMicrophoneNoise sourcein ductPhase shift andamplificationFeedbackcircuitFigure 16.30 Active attenuation diagram